1. Field of the Invention
The present invention relates to a displacement signal output device which covers both linear and rotary encoders.
2. Related Art
The dominant type of displacement signal output devices in use today is a rotary encoder of the transmission type such as that illustrated in FIG. 13. This conventional rotary encoder 181 comprises a rotary disk 183 fixedly mounted on a shaft 182 and provided with a plurality of slits 194 at a constant pitch. Disposed in confronting relation with the rotary disk 183, is a stationary slit plate 186 provided with a plurality of slits 185 disposed at the same pitch as above. The device further includes a light-emitting means 187 and a light-receiving means 188, the former being disposed on the side of the slit plate 186 and the latter on the side of the rotary disk 183.
This arrangement, however, has several disadvantages. First, because the light-emitting means and the light-receiving means are disposed closer to the stationary slit plate and the rotary disk, respectively, it is difficult to implement a displacement signal output device in a compact unit with a flat profile. Furthermore, because the light-emitting means and the light-receiving means are not disposed on the same side, there is imposed a fair restriction on the layout of component members, and it is sometimes difficult to install the device in the required position or build the device into machinery. Moreover, since both the rotary disk and the stationary slit plate should transmit light, there are limitations on the material and construction of these members. For example, only transparent materials or materials in which slits may be formed can be utilized. This, too, narrows the applicability of the device.
As a reflection type displacement signal output device, a linear encoder of the diffraction interference type is also known. In this type of system, laser light is projected on a movable scale provided with a diffraction grating, and the amount of shift in the interference pattern of diffracted light is used to detect the amount of displacement.
Although such an encoder of the diffraction interference type features a high resolution, it unfortunately requires many optical elements, for example three reflectors and a half-mirror, so that the optical system is complicated and the device is of necessity large. In addition, any adjustment calls for a complicated and costly procedure and, moreover, the device is easily susceptible to environmental factors.